In modern integrated circuit (IC) fabrication, layers of material are applied to embedded structures previously formed on semiconductor wafers. Chemical mechanical planarization (CMP) is an abrasive process used to remove these layers and polish the surface of a wafer flat to achieve a desired structure. CMP may be performed on both oxides and metals and generally involves the use of chemical slurries applied in conjunction with a polishing pad in motion relative to the wafer (e.g., pad rotation relative to the wafer). The resulting smooth flat surface is necessary to maintain the photolithographic depth of focus for subsequent steps, and to ensure that the metal interconnects are not deformed over contour steps. Damascene processing requires metal, such as tungsten or copper, to be removed from the top surface of a dielectric to define interconnect structures, using CMP.
As CMP is a chemical-mechanical process, planarization/polishing performance is impacted by the mechanical properties and the slurry distribution ability of the polishing pad. Polishing slurries are formulated to cause passivation layers on the wafer surface, which layers are removed by the mechanical action of the pad. Higher points on the wafer are subject to higher pressure while lower points are protected by passivation and the inability of the pad to reach them. As advanced technology requirements are to be met, low K dielectric materials have to be used. These materials have low mechanical strength and cannot be polished with traditional down force processes.
FIG. 1 illustrates the surface of post-CMP copper wafer 100 polished with a traditional polishing pad alone. The low K dielectric material 102 is capped with a protective dielectric 104 such as silicon dioxide, silicon nitride or silicon carbide. Copper features 106 are etched into the dielectric stack. After polishing with a standard process besides dishing and erosion, damage to dielectric stack is seen in the form of material damage 108 or delamination 110. It is desirable to have a pad that can enable good planarization performance with low or near-zero down force.